While not scientific, I think some ingredients are best left to the idea of artistic interpretation -- or in chef vernacular, "to taste". Onions are the perfect example of this. Like onions just so-so? use medium sized ones. Love 'em? Go large. Hate 'em? Go mini...and dice it really, really small.

NOTE: Skip jumbo. It's a bad category regardless of your affinity for the root. For most all veggies, it's ideal to buy small or medium, because smaller sizes maintain better flavor. The larger ones have often gained their girth through water weight...making for pithy textures and disappointing taste. Think watered-down chicken soup.